This invention relates to a method and apparatus for selectively terminating sealing current in a digital subscriber line (DSL) modem such as an ADSL modem or any XDSL modem used where POTS is the preferred underlying service. More particularly, the present technique comprises providing a mechanism (e.g. a resistor or suitable current sink) to a DSL modem, wherein the mechanism is operable to be dynamically switched between on and off states and to selectively operate under low current conditions. This system allows for provision of DSL (e.g., ADSL or XDSL where POTS is the underlying service) in the absence of plain old telephone service (POTS) and for improved performance.
While the invention is particularly directed to the art of ADSL service, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used in other types of remote communication devices requiring termination of current such as in other XDSL services such as SHDSL, VDSL, VDSL2, ADSL 2+.
By way of background, traditional DSL service was designed to operate in conjunction with Plain Old Telephone Service (POTS) devices. As shown in FIG. 1(a), a network 10 includes a core network 12 in communication with a central office 14. Central office 14 is in communication with a customer through customer premises equipment (CPE) 16. Existing copper lines or loops 18 connect the central office (CO) 14 and the CPE 16. It will be understood by those skilled in the art that the lines or loops 18 comprise a twisted pair of lines (e.g., a tip (T) line and a ring (R) line). In the drawings, the split pair is shown where advantageous for explanation but is not shown in every possible location for ease of viewing.
The central office 14 includes a DSL modem 20, as well as a plain old telephone system (POTS) line card 22. Also shown are high pass filter 21 and low pass filter 23, which facilitate the implementation of the DSL service.
The customer premises equipment (CPE) 16 includes a DSL modem 24, a POTS phone 26 and a personal computer 29 (or the like). Also shown are high pass filter 25 and low pass filter 27, which facilitate the implementation of the DSL service.
The underlying POTS service and equipment shown allows for DC current (that also acts as a sealing current) to flow from the POTS line card 22 disposed within the central office (CO) 14, through the copper lines or loops 18, to a POTS phone 26. The POTS phone 26 terminates the current by completing the circuit. This current is typically run through the copper lines 18 for purposes of powering the POTS phone and also cleaning the copper splices in the copper loops 18. It is known that, absent the DC current, a certain percentage of the copper loops 18 would deteriorate and become obstacles to providing POTS or ADSL service of sufficient quality.
Notably, the DSL modem 24 that is a part of the customer premises equipment (CPE) 16 does not include any mechanism for terminating current. There is no need for such an enhancement to the DSL modem 24 in these circumstances because traditional DSL service is implemented in conjunction with POTS service. As noted above, the POTS phone is operable to terminate current.
However, DSL service is now increasingly being demanded in circumstances where no POTS service is available, e.g., in voice-over-Internet-Protocol (VOIP) networks. Without underlying POTS service, there is consequently no manner of terminating the sealing current—which should be advantageously provided to clean the copper loops on the ADSL lines.
While it is understood that problematic loops in such implementations of DSL service currently constitute only a small percentage of DSL subscriber installations, this small percentage nonetheless translates to a high number of modems. As such, this is a situation that demands a practical solution.
To address the problem currently, service providers simply dispatch service personnel in trucks to the customer premises to install a resistor in the network interface device (NID) at the customer premises. The resistor simulates the off hook condition of the POTS phone 26 and provides a mechanism to terminate the sealing current. To illustrate, referring now to FIG. 1(b), the customer premises equipment (CPE) 16 differs from that of FIG. 1(a) in that underlying POTS service is not available. So, a POTS phone 26 is not present at the premises. Also, the configuration of the filters is different. As shown, a resistor 28 is installed between the tip line and ring line of the twisted pair of loop 18 (in, for example, a network interface device (NID) 17). A low pass filter 27 is also provided.
This approach, though, has its drawbacks. For example, it is expensive to dispatch personnel. Moreover, this approach requires the DC current to be dissipated continuously. This wastes power and, therefore, does not allow for reduction of power consumption. In addition, this approach does not allow for any remote control of the installed resistor.
The present invention contemplates a new and improved system and technique that resolves the above-referenced difficulties and others.